As Greenland melts, ice sheets tremble

Glaciers thawing twice as fast as they were 5 years ago.

Glaciers thawing twice as fast as they were 5 years ago.

June 25, 2006|ROBERT LEE HOTZ Los Angeles Times

JAKOBSHAVN GLACIER, Greenland --Jay Zwally savored the warmth inside the tiny plane as it flew low across Greenland's biggest and fastest-moving outlet glacier. Mile upon mile of the steep fjord was choked with icy rubble from the glacier's disintegrated leading edge. More than six miles of the Jakobshavn had simply crumbled into open water. "My God!" Zwally shouted over the hornet whine of the engines. From satellite sensors and seasons in the field, Zwally, 67, knew the ice sheet below in a way that few could match. Wedged between boxes of scientific instruments, tent bags, duffels and survival gear, the raffish NASA glaciologist with a silver dolphin in one pierced ear was dismayed by how quickly the breakup had occurred. The Greenland ice sheet -- two miles thick and broad enough to blanket an area the size of Mexico -- shapes the world's weather, matched in influence by only Antarctica in the Southern Hemisphere. It glows like milky mother-of-pearl. The sheen of ice blends with drifts of cloud as if snowbanks are taking flight. In its heartland, snow that fell a quarter of a million years ago is still preserved. Temperatures dip as low as 86 degrees below zero. Ground winds can top 200 mph. Along the ice edge, meltwater rivers thread into fraying brown ropes of glacial outwash, where migrating herds of caribou and musk ox graze. The ice is so massive that its weight presses the bedrock of Greenland below sea level, so all-concealing that not until recently did scientists discover that Greenland actually might be three islands. Should all of the ice sheet ever thaw, the meltwater could raise sea level 21 feet and swamp the world's coastal cities, home to a billion people. It would cause higher tides, generate more powerful storm surges and, by altering ocean currents, drastically disrupt the global climate. Climate experts have started to worry that the ice cap is disappearing in ways that computer models had not predicted. By all accounts, the glaciers of Greenland are melting twice as fast as they were five years ago, even as the ice sheets of Antarctica -- the world's largest reservoir of fresh water -- also are shrinking, researchers at NASA's Jet Propulsion Laboratory and the University of Kansas reported in February. Zwally and other researchers have focused their attention on a delicate ribbon -- the equilibrium line, which marks the fulcrum of frost and thaw in Greenland's seasonal balance. The zone runs around the rim of the ice cap like a drawstring. Summer melting, on average, offsets the annual accumulation of snow. Across the ice cap, however, the area of seasonal melting was broader last year than in 27 years of record-keeping, University of Colorado climate scientists reported. In early May, temperatures on the ice cap some days were almost 20 degrees above normal, hovering just below freezing. From cores of ancient Greenland ice extracted by the National Science Foundation, researchers have identified at least 20 sudden climate changes in the last 110,000 years, in which average temperatures fluctuated as much as 15 degrees in a single decade. The increasingly erratic behavior of the Greenland ice has scientists wondering whether the climate, after thousands of years of relative stability, may again start oscillating. For those assessing the effect of global warming, there may be no more perfect place than this warren of red tents on the Northern Hemisphere's largest ice cap. Here, the theoretical effects seen in computerized climate models take tangible form. University of Colorado climatologist Konrad Steffen set up Swiss Camp 155 miles north of the Arctic Circle in 1990 to study the weather along the equilibrium line. As a precaution, Steffen, 54, built the camp on a plywood platform to keep it afloat when the ice turns into summer slush and open lakes before refreezing in the fall. Zwally joined his colleagues there on May 8 in the regular spring migration of scientists to the Arctic. He has been coming to Swiss Camp every year since 1994 and has been studying the polar regions since 1972, monitoring the polar ice through satellite sensors. Eventually he realized he had to study the ice firsthand. The ice sheet seemed such a stolid reservoir of cold that many experts had been confident of its taking centuries for higher temperatures to work their way thousands of feet down to the base of the ice cap and undermine its stability. By and large, computer models supported that view, predicting that as winter temperatures rose more snow would fall across the dome of the ice cap. Thus, by the seasonal bookkeeping of the ice sheet, Greenland would neatly balance its losses through new snow. Indeed, Zwally and his colleagues in March released an analysis of data from two European remote-sensing satellites showing the amount of water locked up in the ice sheet had risen slightly between 1992 and 2002. Then the ice sheet began to confound computer-generated predictions. Most of the computer models on which climate predictions are based did not take the dynamics of the glaciers into account. Contrary to appearances, the monolith of ice is constantly on the move. Swiss Camp has been rafting on the ice stream toward the sea, on average, at about 1 foot every day. Since Steffen pitched the main tents, the camp has moved about a mile downhill. When Zwally started tracking the velocity of the ice with Global Positioning System sensors in 1996, the ice flow maintained a steady pace all year. But he soon discovered that the ice around Swiss Camp had abruptly shifted gears in the summer, moving faster when the surface ice started to melt. By 1999, the ice stream had almost tripled its speed to about 3 feet a day. In an influential paper published in the journal Science, Zwally surmised that the ice sheets had accelerated in response to warmer temperatures, as summer meltwater lubricated the base of the ice sheet and allowed it to slide faster toward the sea. In a way no one had detected, the warm water made its way through thousands of feet of ice to the bedrock -- in weeks, not decades or centuries. So much water streamed beneath the ice that in high summer the entire ice sheet near Swiss Camp briefly bulged 2 feet higher, like the crest of a subterranean wave. "This meltwater acceleration is new," Zwally said. "The significance of this is that it is a mechanism for climate change to get into the ice." To better track the seasonal movements, Zwally and Steffen set up two new GPS stations around Swiss Camp, while a team led by University of Vermont geophysicist Tom Neumann erected an additional 10 GPS sensors to map the changing velocity of the local ice. At the same time, University of Texas physicist Ginny Catania pulled an ice-penetrating radar in a search pattern around the camp, seeking evidence of any melt holes or drainage crevices that could so quickly channel the hot water of global warming deep into the ice. To her surprise, she detected a maze of tunnels, natural pipes and cracks beneath the unblemished surface. No one knows how much of the ice sheet is affected. In the Swiss Camp laboratory tent, Rial moved his finger along the jagged seismic trace displayed on his iBook screen, documenting activity caused by accelerating ice flow. The signal had been detected by the 10 sensors he had placed around the camp six days before. The ice sheet was trembling.